One of the good things Enterprise did was explain why Klingons don't have ridges in TOS. The real reason was budget, TOS had a very low budget. Browned skin and Fu-Manchu moustaches were about all they could afford. The reason Enterprise cooked up was a virus spawned from a sick Klingon who was given a genetic cocktail made from human augment DNA. It mutated with the virus and made a plauge that was killing millions of Klingons. With Dr. Floxes help they found a cure. But it left anyone infected with no ridges.